There will be opportunities for cheap deals closer to the Alaska cruise, however you will not be sure of what ship, cruise line or dates those deals will be for. If you find a TA that is willing to go to bat for you and try to get you a price reduction if the price does drop booking earlier is better than waiting for a last minute deal. Many specials for Alaska, if there are any, will start coming out around January or February for the months of May and June. If you want a specific room on a ship you are best off booking early.
As for kids on an Alaskan cruise, there will be some kids but Alaska usually draws people that are more interested in wildlife or the view of mountains and glaciers. An Alaska cruise will not have as many kids as would be on a Caribbean cruise. An Alaskan cruise is not a high activity cruise it will have people standing by the rails or near the windows watching the scenery. Alaska tends to have a few more older people onboard than you would find on a Caribbean cruise.
One of the greatest things about cruising Alaska is that you never know what you will see or where you will see it. You may be lucky while near the glaciers and see a huge calve and in that case the site and sound will keep you in awe. You may be fortunate to see some humback whales just rolling along, breaching or bubblenet feeding and again you will be kept in awe. You may see a pod of Orca skimming along the water with their high dorcel fins cutting through the water or see them leap out of the water and again you will be in awe. You may be fortunate to see the dahls porpiose as they play off the wake of the ship. You may see bears or moose and possibly mountain goats.
You will see eagles without a doubt as well as many other species of birds. Chances are good you will see sea otters along your cruise.
While near the glaciers pray for some clouds because the glaciers will look a little bluer if there is some cloud cover. You can have great flight tours or raft down rivers. You can have fun kayaking or fishing. You can see trees that are hundreds of years old yet only a few feet tall due to harsh winters back in the Yukon. You can see some beautiful totem poles and native artwork.
Alaska has many wonderful things to do and see and even if it does not turn out to be your favorite place to cruise I am more than willing to bet it is a place you will not forget.